Spinning LED Display
Andrew Petersen + JP Skeath
Purpose of the Project
Demonstrate Proficiency with precision timing, the HCS12 microcontroller, and
mechanical interfacing
Design and interface a system that has a stator and rotor
Reproduce a modern method of displaying data
Background Information
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Platform gives form and support
LED’s turn on and off for a set
period of time
Motor rotates the platform faster
than your eye “refresh rate”
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Higher RPM increases the resolution
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Faster blinks increase the sharpness of
the image
Turning specific LED’s on at
precise timings provide the
message
http://hackedgadgets.com/2010/05/03/rotating-led-globe/
Physical Construction
16 LED’s on 2 vertical panels
1 300 RPM DC motor
HCS12 Nanocore module standalone
9V battery supply
LED driver with constant current sink
(We used MAX6969ANG)
Letter Patterns
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38 Supported characters
A-Z, 0-9, :, .
5x8 pixels each
Write each column one at a time
to display with time delay in between
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LED’s correspond to vertical resolution
Timing between displayed LED
columns determines horizontal spacing
All patterns stored in #definitions in
C-code
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Display Timing
-By utilizing each LED panel we effectively
double the framerate compared to motor
rotation speed (2*300rpm -> 10Hz theoretical)
-Matching programmed half rotation
time (T_hrot) results in stable image
-Variations in half rotation timing cause
characters to “spin” around display
Display Timing cont.
Writing data to LED panels:
-Utilizes SPI interface
-Uses MAX6969 LED Driver
-Provides 16 current sinks. 8 used for
each panel
-This functionality encapsulated in DriveLED()
function
Pseudocode of Program
Main Program
Configure DDRT and DDRM
Initialize SPI
loop forever
print string to display
end
Printing Character
write
delay
write
delay
...
write
delay
delay
1st column corr. to char
T_b
2nd column corr. to char
T_b
5th column corr. to char
T_b
T_b
Pseudocode cont.
Printing a string
for each char in string
print char to one panel on display
end
delay (half rotation time)- (elapsed string time)
for each char in string
print char to other panel on display
end
delay (half rotation time)- (elapsed string time)
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Utilize free running counter to keep
track of elapsed time to display
message on 1st panel
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Subtract elapsed time from half
rotation time to determine when to
display message again on 2nd panel
C Code Snippets
switch(type){
case(0):
driveLED((unsigned int)(_A0));
DelayuSec(t);
driveLED((unsigned int)(_A1));
DelayuSec(t);
driveLED((unsigned int)(_A2));
DelayuSec(t);
driveLED((unsigned int)(_A3));
DelayuSec(t);
driveLED((unsigned int)(_A4));
DelayuSec(t);
driveLED(0x0000);
DelayuSec(t);
break;
...
void driveLED(unsigned int data){
PTT = PTT & 0xFE; //Sets PT0 (LE) to low
putcharSPI((char)(data >> 8));
putcharSPI((char)(data & 0x00FF));
PTT = PTT | 0x01; //Sets PT0 (LE) to high
PTT = PTT & 0xFE; //Sets PT0 (LE) to low
}
printStrLED("HELLO");
Current Results
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Can display characters
Currently limited to 4 characters (more in advanced version)
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Due to half rotation delay calculation, 4 characters fit comfortably in a semicircle on
our display
NOTE: Image continuous with
eyes. Video camera caused
disruption due to rolling shutter.
Difficulties Encountered
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Brightness of the LEDs and visibility of project in general
Nanocore module failure during one test (unsure of cause)
Power supply for the motor(stall current of about 2A. Start of .7 A)
Noisy Power supply DC voltage (fluctuations cause different RPMs)
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Coupling Capacitor solution
Weight Constraints
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Wobbling
Possible loss of RPM
Questions?